Descriptions of units, abbreviation, terminology, etc. used throughout the present disclosure are summarized in Table 1.
Aromatic compounds can be alkylated to form different alkylated aromatic products. One that has particular value is para-xylene (also referred to as p-xylene). Para-xylene is a valuable substituted aromatic compound due to its great demand of approximately 30 million metric tons per year for further oxidation to terephthalic acid, a major component in forming polyester fibers and resins. Commercially, para-xylene is generally produced by one of two methods. The first and most prominent, from which more than about 80% of the para-xylene is derived, involves hydrotreating of naphtha (catalytic reforming) and/or steam cracking of naphtha or gas oil. The second method, from which more than about 15% of the para-xylene is derived, involves toluene disproportionation (TDP) to produce benzene and mixed xylene (it is noted that mixed xylene comprises o-, m-, and p-xylene), where the p-xylene is generally present in an amount of about 20 to 25% based on the total xylene. In a shape selective toluene disproportionation (also known as STDP) p-xylene content in mixed xylene can be greater than 80%, specifically, greater than or equal to 90%, more specifically, greater than or equal to 95%. Para-xylene can then be further separated from mixed xylene produced by catalytic reforming or naphtha cracking or toluene disproportionation.
Alkylation of toluene with methanol, which is also known as toluene methylation, has been used in laboratory studies to produce mixed xylene. Generally, a thermodynamic equilibrium mixture of o-, m-, and p-xylene can be formed from the methylation of toluene, as is illustrated by the following reaction:

When a shape selective catalyst, such as a modified ZSM-5 zeolite catalyst is used for toluene methylation, the reaction can produce mixed xylene with the p-xylene isomer being greater than or equal to 80%. However, the byproducts of toluene methylation, including C9+ aromatics are produced due to secondary alkylation of xylene and alkylation of toluene with C2+ compounds. Such C9+ aromatic byproducts are less valuable than the xylene and such byproduct formation can make the toluene methylation commercially unattractive. Furthermore, there are a number of technical hurdles (e.g., catalyst deactivation, low methanol selectivity) for toluene alkylation to be commercially attractive and an improved method of preparation of mixed xylene is desirable.